The present invention is related to the alignment and registration of components onto substrates in a machine placement environment. More particularly, the present invention is directed to a method for utilizing a high-resolution placement head in conjunction with a relatively low-resolution placement system to obtain high-resolution placement of components onto substrates.
Robotic placement systems (robots) are used widely throughout industry. For example, they are used to place electronic components on printed circuit boards (PCBs) in the electronics assembly industry, they are used to build automobiles and other mechanical equipment, they are used in biotechnology to place samples in test cells and to efficiently process experiments. Most robotic placement systems provide at least four axes of control: the X- and Y-axes defining the horizontal plane, the Z or vertical axis, and the T or rotational axis.
The robot""s Z-axis that can move up and down often holds a tool, gripper, magnet or vacuum nozzle which can acquire components. This is often referred to as a pick-up head. While a component is mounted to the pick-up head, several process steps can be performed on the component before the component is released. Generally, the cost of a robot axis is highly influenced by the positioning accuracy and positioning repeatability required of the axis. A highly accurate axis is generally much more expensive than a less accurate axis. In the past, the accuracy of a robot axis had to be much more accurate than the accuracy requirement for a particular process to be performed by the axis. If, for example, a placement process has to be accurate to within +/xe2x88x9220 microns, the robot""s axis position accuracy must generally be about ten times greater, i.e., +/xe2x88x922 microns. Such accuracy requirements result in extremely expensive equipment. To achieve an axis with a positional accuracy of +/xe2x88x922 microns, air-type bearings fabricated out of ceramic components are often required, particularly for large spans used in robotic assembly equipment in the electronics industry. The larger such bearings are, the more they cost. It would, accordingly, be highly desirable to provide a way to avoid this high cost while achieving desired high accuracy placement by other means.
A placement robot having a given positional resolution accuracy (e.g., in the X and/or Y directions of a horizontal plane) positions a component for placement at a selected location on a target substrate by placing it at a first position displaced in the Z (vertical) direction over the target substrate disposed within a first resolution accuracy (in X and/or Y directions) of the selected location. A micro alignment pick-up head (MAP) holds the component. The MAP includes a retention member that may be a vacuum pick-up nozzle for component retention and has two main parts: an upper body and a lower body. The upper body and the lower body are separated from one another by an air bearing which provides near-frictionless performance and allows the lower body a range of motion relative to the upper body. A pressurized gas line to the MAP provides pressurized air to the air bearing. The nozzle with the component attached therefore has a range of motion within the air bearing which exceeds the positional resolution accuracy of the basic placement robot. The MAP may be in one of two states. The first state is referred to as the xe2x80x9cattachedxe2x80x9d state. In this state the air bearing is disabled, as by inverting the pressure on the air bearing to vacuum, and the upper body and lower body are therefore locked together and move as one. The second state is the xe2x80x9cloosexe2x80x9d state. In this state the air bearing is active and the pick-up head nozzle may be moved freely with no appreciable friction. Micropositioning actuators may be coupled to the MAP""s upper body to contact the lower body so as to provide fine positioning in the X and/or Y directions. Alternately, by leaving the lower body in the xe2x80x9cloosexe2x80x9d state, the lower body may be brought into accurate alignment as by the application of surface tension forces applied by melting solder, or by other suitable means. Accordingly, once initially positioned within a first resolution accuracy of the selected location, additional positioning may be performed to bring the component to within a second resolution accuracy of the selected location, the second resolution accuracy being less than the first resolution accuracy. Once thus positioned, the component may be brought toward or into contact with the substrate, as desired, by activation of the Z-axis actuator in a conventional manner.